The present invention relates to a pneumatic tire for promoting higher fuel economy for a vehicle. More specifically, the invention relates to a pneumatic tire capable of reducing rolling resistance without damaging tire performance such as running performance on a wet road surface (wet performance) or wear resistance.
Generally, rubber of a large hysteresis loss is used for a cap tread of a tire so as to secure wear resistance and wet performance. On the other hand, for reducing tire rolling resistance so as to increase fuel economy, rubber of a small hysteresis loss must be used.
However, if rubber mixed for lower fuel costs, i.e., rubber of a small hysteresis loss, is used for reducing rolling resistance, a reduction inevitably occurs in tire performance such as wet performance or wear resistance. Thus, it has been considered that a characteristic of rolling resistance and a characteristic of wet performance or wear resistance are mutually incompatible.
It is an object of the present invention to provide a pneumatic tire capable of reducing rolling resistance without damaging tire performance such as wet performance or wear resistance.
In order to achieve the foregoing object, provided is a pneumatic tire according to an aspect of the present invention which comprises a carcass layer provided between a pair of left and right bead sections; left and right side wall sections interpolated for connecting the pair of left and right bead sections with a tread section to be continues; and at least two belt layers provided in an outer peripheral side of the carcass layer in the tread section, each of the belt layers being composed of a plurality of reinforcing cords coated with coating rubber. This pneumatic tire is characterized in that storage elastic modulus Exe2x80x2 of the coating rubber is set in a range of 15.0 to 40.0 MPa, its loss tangent tan xcex4 is set in a range of 0.1 to 0.25, interlayer rubber is provided between end parts of the belt layers, the interlayer rubber being specifically set in each of both end parts thereof in a tire width direction, and elongation to break Eb of the interlayer rubber is set in a range of 400 to 700%.
We conducted earnest studies with a view to suppressing deformation of the tread section. As a result, we discovered that an increase of coating rubber storage elastic modulus Exe2x80x2 for the belt layer in the foregoing range and suppression of deformation of the belt layer or the tread section adjacent to the belt layer were effective for reducing rolling resistance. Thus, it is not necessary to use, for a cap tread, rubber of a small hysteresis loss mixed for lower fuel costs. Therefore, rolling resistance can be reduced without damaging tire performance such as wet performance or wear resistance.
Even if coating rubber storage elastic modulus Exe2x80x2 for the belt layer is increased, an effect of reducing rolling resistance will be insufficient if its loss tangent tan xcex4 is large. Accordingly, a coating rubber loss tangent tan xcex4 for the belt layer must be set small in the foregoing range.
If coating rubber storage elastic modulus Exe2x80x2 for the belt layer is large, an interlayer movement between the end parts of the belt layers will be restricted and thus a failure will easily occur in this portion. Accordingly, elongation to break Eb of the interlayer rubber provided between the end parts of the belt layers must be set large in the foregoing range, and fatigue resistance to interlayer shearing deformation in the end parts of the belt layers must be increased.
In addition, storage elastic modulus Exe2x80x2 of side wall rubber for each of the side wall sections should preferably be set small in a range of 2.0 to 3.0 MPa. By setting small storage elastic modulus Exe2x80x2 of the side wall rubber, deformation easily occurs in the side wall section which gives only a small effect on rolling resistance, and thus a synergistic effect of suppressing deformation of the tread section can be provided. For providing an effect of sufficiently reducing rolling resistance, a loss tangent tan xcex4 of the side wall rubber should preferably be set small in a range of 0.07 to 0.15.
Furthermore, a height of a bead apex arranged in each of the bead sections should preferably be set in a range of 10 to 35% of a tire section height SH. By setting low a height of the bead apex, deformation easily occurs in the bead section, and thus a synergistic effect of suppressing deformation of the tread section can be provided. For providing an effect of sufficiently reducing rolling resistance, a loss tangent tan xcex4 of bead apex rubber for the bead apex must be set small in a range of 0.1 to 0.25.
In order to achieve the foregoing object, provided is a pneumatic tire according to another aspect of the present invention which comprises a carcass layer provided between a pair of left and right bead sections, each of both end parts of the carcass layer in a tire width direction being turned up around a bead core from the inside of a tire to its outside; and a bead apex arranged in an outer peripheral side of the bead core. This pneumatic tire is characterized in that at least a 30% area of a section area of the bead apex positioned in a range of 20 to 35% of a tire section height SH is made of low tan xcex4 rubber which is set in a range of 25 to 75% of tan xcex4 of base bead apex rubber for a bead apex main body.
We conducted earnest studies with a view to effectively suppressing energy losses concentrated in the bead sections. As a result, we discovered that the occurrence of energy losses in each of the bead sections concentrated in an area from an upper part to a halfway part of the bead apex, especially in its outside, positioned in the range of 20 to 35% of the tire section height SH. Then, we discovered that by selectively arranging low tan xcex4 rubber only in a portion of the bead section where the occurrence of energy losses concentrated, it was possible to effectively suppress energy losses concentrated in the bead section and thereby reduce rolling resistance. Therefore, since it is not necessary to use, for a cap tread, rubber of a small hysteresis loss mixed for lower fuel costs, rolling resistance can be reduced without damaging tire performance such as wet performance or wear resistance. Moreover, even if handling stability is improved by using rubber of high hardness and high tan xcex4 for base bead apex rubber for a bead apex main body, an increase of rolling resistance will be prevented by locally arranging low tan xcex4 rubber as described above. Accordingly, reduced rolling resistance and handling stability can be provided simultaneously on a high order.
In addition, a turned-up edge of the carcass layer should preferably be arranged to be outside the range of 20 to 35% of the tire section height SH. By arranging the wound-up end of the carcass layer to be outside the range where the occurrence of energy losses concentrates, durability can be increased.
In the present invention, storage elastic modulus Exe2x80x2 was measured by using a visco-elastic spectrometer (made by Iwamoto Works) under the following conditions: frequency 20 Hz; initial strain 10%; dynamic strain xc2x12%; and temperature 60xc2x0 C. A loss tangent tan xcex4 was measured by using the visco-elastic spectrometer (made by Iwamoto Works) under the following conditions: frequency 20 Hz; initial strain 10%; dynamic strain xc2x12%; and temperature 60xc2x0 C.